1. Field of the Invention
This invention relates to a signal reproducing circuit for reproducing angular modulated signals recorded on a recording medium in a video tape recorder (hereinafter referred to as VTR) or the like, and more particularly the invention provides said type of angular modulated signal reproducing circuit which can prevent the so-called "signal inversion" induced by an abrupt change of the signal level to obtain a high-quality reproduced output signal free of S/N deterioration.
2. Description of the Prior Art
In the video recording and/or reproducing apparatus such as VTR, recording on a magnetic recording medium or reproduction of the recorded information is usually accomplished by utilizing angular modulation, particularly frequency modulation. In such magnetic recording and reproducing system, particularly the one in which the relative speeds of the magnetic recording medium and the magnetic transducer head are low and also the track is very narrow in width, it is possible to attain an improvement of S/N by treating the frequency modulated video signal with high deviation and high emphasis. However, in such signal recording or reproduction with high deviation and high emphasis, particularly in case of reproducing an image which undergoes an abrupt change in luminance signal level of the picture signal, for example, an image which changes from black to white in the horizontal scanning direction on the screen, there may take place "signal inversion", that is, horizontal black streaks 4 are formed in the white region 2 from the boundary line 3 where the black region 1 changes into the white region 2 as shown in FIG. 1.
The reasons for causing such signal inversion may be explained as follows. The video signal, such as shown in FIG. 2A, of an image changing from black to white in the horizontal scanning direction on the screen is, in a VTR device, pre-emphasized before frequency modulation, so that such video signal may have an over-shoot at the portion where the black level changes into the white level as shown in FIG. 2B. In a VTR of a low carrier frequency FM system, when said recording video signal is angle-modulated by an angular modulator, the carrier level at said portion of over-shooting is attenuated, since such carrier level is high at said portion. Also, there is produced an amplitude modulation component at said portion of an over-shoot due to transient distortion in the signal transmission system or phase rotation caused by the high-pass filter, and it follows that the zero-crossing shift of the reproduced angle modulated signal, that is, the crests and troughs of the carrier wave do not cross the detecting line as shown in FIG. 2C. Therefore, when said reproduced angle modulated signal is wave shaped through a hard limiter in the signal reproduction system, there is induced dropout of pulse as shown in FIG. 2D to make it impossible to effect high-fidelity demodulation, and this leads to "signal inversion", that is, the region where the black level of the final reproduced video signal has changed into the white level is still regarded as black level as shown in FIG. 2E.
Such signal inversion can be prevented by emphasizing the high frequency component of the carrier before limiting the amplitude of the reproduced angle modulated signal. Such high frequency emphasis elevates the carrier level at the over-shoot portion of the reproduced angle modulated signal, so that no dropout of pulse is caused even if amplitude limiting is made in the later stage.
Generally, however, in a VTR of the low carrier frequency FM system, S/N of the high level component of the carrier is poor, and the more emphasized is this high frequency component, the more deteriorated is S/N of the reproduced video signal obtained after demodulation. Thus, there is a dilemma that an attempt to prevent said signal inversion leads to deterioration of S/N and an attempt to better S/N leads to the signal inversion.
A circuit such as shown in FIG. 3 has been proposed as a magnetic recording and reproducing system for obtaining high-quality demodulated output signal by eliminating said dilemma to allow prevention of the signal inversion without causing S/N deterioration.
In the magnetic recording and reproducing system shown in FIG. 3, an input video signal (Y.sub.in signal) with a frequency distribution condition such as shown in FIG. 4A is fed to the signal input terminal 10, and this input video signal is angle modulated by an angular modulation circuit 11 including a pre-emphasis circuit and thereby converted into an angle modulated signal (Y-FM signal) having an upper side band and a lower side band such as shown in FIG. 4B, and such signal is recorded on a magnetic tape 13 through a recording magnetic head 12. Here, it is assumed that an input video signal (Y.sub.in signal) of a waveform shown in FIG. 2A is supplied to said signal input terminal 10. The angle modulated signal (Y-FM signal) recorded on said magnetic tape 13 is picked up by the reproducing magnetic head 14, converted into a reproduced angle modulated signal (Y.sub.out -FM signal) with its usually upper side band being suppressed as shown in FIG. 4C and output through a reproducing amplifier 15. The waveform of the reproduced angle modulated signal (Y.sub.out -FM signal) obtained through said reproducing amplifier 15 is as shown in FIG. 2C. Said reproduced angle modulated signal (Y.sub.out -FM signal) is limited to a slight degree of a soft limiter circuit 16 with limiter gain of the order of 10-12 dB, and as a result, the upper side band is regenerated by the lower side band component of the Y.sub.out -FM signal as shown in FIG. 4D. The reproduced angle modulated signal (Y.sub.out -FM signal) having its upper side band regenerated as above presents a waveform with its zero-crossing section shaped as shown in FIG. 2F, and then this signal is passed through a high emphasis circuit 17 having a frequency characteristic such as shown in FIG. 4E whereby it is changed into a frequency distribution pattern with its high frequency signal component emphasized as shown in FIG. 4F and then is supplied to an angle demodulation circuit 19 through a hard limiter circuit 18 with limiter gain of the order to 40-50 dB. Here, the S/N ratio of the upper side band of the reproduced angle modulated signal (Y.sub.out -FM signal) obtained through said reproducing amplifier 15 is bad, but the upper side band of the reproduced angle modulated signal (Y.sub.out -FM' signal) can be regenerated through the soft limiter circuit 16, so that the total S/N ratio of the signal is improved. Therefore, no S/N deterioration of the signal is caused even if the high frequency component is emphasized by the high emphasis circuit 17. Thus, supposing here that an input video signal (Y.sub.in signal) with a waveform such as shown in FIG. 2A is being supplied to the signal input terminal 10, then the reproduced angle modulated signal (Y.sub.out -FM signal) of FIG. 2C obtained through the reproducing amplifier 15 is limited by the soft limiter circuit 16 to obtain a signal (Y.sub.out -FM' signal) with a waveform such as shown in FIG. 2F, and further the high frequency component of this signal is emphasized by the high emphasis circuit 17 to allow correct zero-crossing as shown in FIG. 2G and then limited by the hard limiter 18. The output signal obtained through the hard limiter 18 can be correctly demodulated by a demodulator 19 without causing any dropout of pulse as shown in FIG. 2H. FIG. 2I shows the waveform of the demodulated output signal obtained after angle demodulation by said demodulator 19.
It is to be also noted that in the magnetic recording and reproducing system of the construction such as shown in FIG. 3, because of low carrier level at the section where the luminance signal level of the image changes sharply, the third harmonic component of the carrier of the reproduced angle modulated signal limited by the soft limiter circuit 16 tends to be emphasized by the high emphasis circuit 17, and if the gain of said high emphasis is too large, said third harmonic component comes to cross the detecting line to cause the signal inversion.
For instance, in case of recording or reproducing an input video signal which undergoes a change from white level to black level and also involves under-shoot by pre-emphasis as shown in FIG. 5A, the reproduced angle modulated signal such as shown in FIG. 5B obtained from the reproducing amplifier 15 is amplitude-controlled as shown in FIG. 5C through the soft limiter circuit 16, but if the gain of the high emphasis circuit 17 is too large, the third harmonic component of the carrier of said reproduced angle modulated signal is emphasized as shown in FIG. 5D, so that if this signal is subjected to amplitude limiting by the hard limiter circuit 18, there is produced an unnecessary pulse P.sub.e as shown in FIG. 5E and such unnecessary pulse P.sub.e is judged as white level by the demodulator 19, causing inversion due to white over-modulation.